I've seen the 150wpc at 4 Ohms somewhere, too. The amps per channel figure is close to double by your specs. My 19 is original & I've always used fuses as a measure of care. I may try recapping all but the PS filter cans,any thoughts?
Yes I have some thoughts. I have done the Driver Boards already but I have been hesitant to post pictures them because I have learned some things since and I am going to make a major change. Plus I forgot to take a picture before I put them back in place. Basically C1 & C6 are the 2 most important caps on the whole board. C1 is the input cap and C6 is the negative feedback cap. I would use either the Mundorf E-Caps
http://www.partsconnexion.com/capacitor_ele_mundorf_bipol.html or the Jensens axials [100uf 150V & 470uf 150V].
http://www.partsconnexion.com/capacitor_ele_jensen_axial.html Either one will take some skill to mount either in an axial direction, or in a stand mount direction. I can post some pictures of a stand mount of an axial cap if you have never done it.
There are 2 more alternatives. 1. An easier approach and a good one would be to simply get some Nichicon Muze radials. They are more powerful then the Elnas in my experience. Then bypass them with a .01 REL RTE
http://www.partsconnexion.com/capacitor_film_reliablecap.html, which are about $6 ea. Even if you use the Jensens or the Mundorf I would bypass them with this cap and also use the RTE to replace the .01 ceramics on the board. The RTE rated "better than the SoniCap Platinum (which costs four times as much) with its smoothness and lack of glare." In the Great Capacitor Shoot-Out
http://www.laventure.net/tourist/caps.htm . I have used the RTE's in my phono stage and I love them. I also used them to replace the ceramics on the 19.
Word of advice for those who are wondering like I was - Do not increase the capacitance for C1, or C6 because they determine the amplified frequency range and it is already full bandwidth. This is where I made a slight misjudgement in increasing C1 to 220uf, which is o.k. - it was just overkill. Increasing the bandwidth too much can cause oscillations that could harm the amplifier. A typical value for an input cap is 4.7uf to 10uf. HK used a 100uf! However, the amp was designed to be full bandwidth and can handle this value. Just don't go any higher here or anywhere for that matter other than the main capacitor bank. I was told the main capacitor back could handle 22K but I couldn't find a 22K that would fit. You will notice the Driver Assay board has a .1uf and its role is as a zobel capacitor, which is only there to counteract speaker inductance and stabilize the amp against stray frequencies that may cause catastrophic oscillations. It doesn't contribute to sound and although I upgraded mine it is not necessary.
[Note: I would like to thank everyone over at DIYAudio who helped me out tremendously to understand the circuit diagram.]
I originally used a Jensen 220uf for the input cap and bypassed it with a Vitamin Q because I was looking for a vintage tone sound - which I really like the vitamin Qs. I used a Nichicon for the 470uf and bypassed it with a Vishay 1837. After having done that I learned about the the role of C1 & C6 in how they determine bandwidth and now see it is more important than I originally anticipated. Also, in the interim I ended up upgrading the power supply of my HK 17 Preamp to all Blackgates
http://www.audiokarma.org/forums/showthread.php?t=395148&page=5 and they are as good as a film cap IMHO! Therefore I am now hunting down Blackgates for C1 & C6. If you can afford them there is nothing better that I would recommend. I am not even going to bypass them because I read that it can ruin their sound.
I will post some pics tonight of my first attempt at the driver boards. Also C16 is tricky too to upgrade and I can talk more about that later.